The present invention relates to a catalyst for the dehydrogenation of C.sub.6 -C.sub.15 paraffins and to a process for making such catalysts.
The dehydrogenation of hydrocarbons and in particular of paraffins is performed on an industrial scale. The dehydrogenated products are required for the production of a variety of chemical compounds such as, for example, detergents, components for gasoline, and pharmaceutical products. The dehydrogenation of normal paraffins with 6-15 carbon atoms to give the corresponding monoolefins is of particular importance for the production of detergents. Monoolefins are used for alkylating aromatic compounds such as, for example benzene, to give the corresponding linear alkylbenzenes.
Dehydrogenation of paraffins is performed in the presence of a suitable catalyst under dehydrogenation conditions. The paraffins may make contact with the catalyst in a fixed bed, in a fluidized bed or in a moving bed. A fixed bed catalyst system is preferably used where the reactant to be dehydrogenated is first preheated to the required reaction temperature and then passed over the fixed bed catalyst. Hydrocarbons are preferably dehydrogenated in the vapor phase. The temperatures required for this are in the range between 300 and 900.degree. C.
Catalysts used for dehydrogenation on an industrial scale contain platinum on aluminum oxide as support. The catalysts generally also contain several promoters, for example tin and/or indium. When dehydrogenating normal paraffins the objective is to produce straight-chain monoolefins. Therefore, side reactions or consecutive reactions to give isoolefins, isoparaffins or alkylated aromatic compounds should be avoided. In practice, these unwanted reactions are suppressed by adding an alkali metal which modifies the catalytic activity in an appropriate manner.
U.S. Pat. No. 4,486,547 (which is incorporated by reference in its entirety) discloses, for example a catalyst composition which contains a platinum group component, a tin component, an indium component, and an alkali metal or alkaline earth metal component on a porous support. In the examples, only catalysts with the alkali metal lithium are described.
Due to the additional indium component, the catalyst system in U.S. Pat. No. 4,486,547 exhibits greater stability of catalyst activity than comparable catalysts without indium. For industrial use of this type of catalyst system, however, further improvements in catalyst activity and in particular in long-term stability of the catalyst system are desirable.